1. Field of the Invention
This invention relates to a method of producing steel pipe material for an oil well, and particularly to steel pipe material excellent in resistance to sulfide stress corrosion for a sour oil well or a gas well.
2. Description of the Prior Art
High strength and high corrosion resistance are required from the steel pipes for an oil well or gas well depending upon the depth, degree of acidity and the like. Recently, there has been a tendency that the depths of oil wells everywhere in the world are increased more and more, with the result that steel pipes of high strength have been demanded to be supplied. At the same time, with the oil wells of acidity having high content of sulfides such as hydrogen sulfide, properties resistant to stress corrosion of said sulfides are required from the steel pipes.
There has heretofore been known such a relationship that said resistance to sulfide stress corrosion is inversely proportional to the increase in strength of the steel, i.e. the resistance to sulfide stress corrosion is decreased with the increase in strength of the steel pipe.
There has been disclosed an invention of Japanese Patent Application Laid-Open (Kokai) No. 52114/77 as the prior art of producing steel pipe material of high strength and having resistance to sulfide stress corrosion for an oil well. This steel is introduced thereinto with Boron (B). Because B loses its effects when forming a nitride, Titanium (Ti) having a strong trend of producing a nitride is added at the same time. However, Ti has a high affinity with oxygen, whereby the amount of non-metallic inclusions are increased, thus presenting such a disadvantage that machinability of the steel pipe for an oil well in the threading process is considerably deteriorated.